The term “large crystal products” as meant herein, does not intend to reflect any absolute value or value range as such, but—in the context of this application—it always is related to a predetermined average crystal size (generally measured along the crystal diameter) for any particular crystalline product prepared. For crystals of each specific product to be called a “large crystal product” in the meaning of this application, accordingly, the predetermined average crystal size should be within a certain range of minimum and maximum diameter (dmin and dmax), for instance of between 1 and 5 mm of diameter. Moreover, the percentages of crystals of the crystalline product prepared, and having a diameter smaller than dmin, respectively larger than dmax, preferably should not exceed a specific value as will be determined by the product specifications. Generally such percentages of crystals having a diameter smaller than dmin, respectively larger than dmax, will be at most 20% by weight of the crystal product, and more preferably both are at most 15% by weight, even more preferably both are at most 10% by weight, most preferably at most 5% by weight. Simultaneously, the percentage of crystals having a diameter in the range between dmin and dmax will generally be at least 60% by weight, more preferably at least 70% by weight, even more preferably at least 80% by weight and most preferably at least 90% by weight. If, for a specific product, the combination of these criteria (namely the criteria (i) average crystal size within range of dmin to dmax; (ii) percentages of crystals smaller than dmin, respectively larger than dmax; and (iii) percentage of crystals in the range between dmin and dmax) is not met in the widest sense (i.e. at most 20% by weight for each of the smaller than, respectively larger than, aspects of (ii); and at least 60% by weight for (iii)), then the product cannot be referred to as a “large crystal product” in the sense of this application.
For crystalline ammonium sulphate, for instance, the crystals of the market specifications for the large crystal product have an average crystal diameter in the range of from dmin=1.0 mm to dmax=3.5 mm, and meet the criteria of (ii) and (iii). If these crystals tend to have a big and rough appearance, they also may be called granular, even if they have been obtained by crystallization and not by any of the granulation techniques known to the skilled man. Ammonium sulphate large crystal products are particularly suitable for use in fertilizers. Market specifications of ammonium sulphate large crystal products may be different depending on the market where the product is used. For instance, in the European market, specifications for ammonium sulphate large crystal products are such that at most 10% by weight has a crystal diameter smaller than 1.4 mm, respectively at most 10% by weight has a crystal diameter larger than 3.35 mm, with the proviso that at least 90% by weight has a diameter in the range of from 1.4 to 3.35 mm. For the USA market the average value of the ammonium sulphate large crystal products generally may be somewhat smaller than in Europe; in the USA at most 10% by weight has a crystal diameter smaller than 1.0 mm. Nevertheless, in the context of this application, large crystal products of ammonium sulphate are already such products (though being outside the criteria of the market specifications) which meet the abovementioned combination of the criteria of (i) average crystal size within range of dmin to dmax; and (ii) percentages of crystals smaller than dmin, respectively larger than dmax; and (iii) percentage of crystals in the range between dmin and dmax in the widest sense. Narrower specification ranges then can be met by subjecting the large crystal product to a subsequent sieving step.
Weight percentages of crystals below a certain diameter (or conversely above a certain diameter) can be determined according to standard methods known to the skilled man, e.g. by sieving crystalline products over screens having different mesh size openings. See for instance the techniques described in Perry's Chemical Engineers Handbook, 6th edition, 1984, pages 21-13 until and including 21-18.
As used in this application, the term “fluid bed crystallizer” intends to describe any type of crystallizer wherein the crystal slurry present is not homogeneously mixed throughout all of the liquid contents of the crystallizer. Accordingly, solids are not evenly spread throughout the height of the liquid column in the crystallizer. Generally some kind of “crystal bed” can be observed in these crystallizers, i.e. a part of the volume of the liquid contents of the crystallizer containing almost all of the crystals in the crystallizer (this is the lower part of the crystallizer liquid content); another part of the volume of the liquid contents of the crystallizer (this is the top part of the crystallizer liquid content) contains only very few crystals. In normal operation of a fluid bed crystallizer, external circulation is carried out from this top part of the crystallizer content, thereby providing an external circulation stream which generally is (almost) crystal-free. The height of the crystal bed (in cm) in the crystallizer is calculated from the bottom of the crystallizer till the level of the boundary surface between the part of the volume of the liquid contents of the crystallizer containing almost all of the crystals and the part of the volume of the liquid contents of the crystallizer containing only very few crystals. Various types of fluid bed crystallizers are known to the skilled man, for instance so-called Oslo-crystallizers and Krystal-crystallizers as are described in Crystallization, Third Edition, pages 338, 345 and 351, by J. W. Mullin, Butterworth-Heinemann (1993).
Further, as used in this application, the term “means for mechanically impacting of crystals” indicates any type of apparatus, equipment or part thereof which leads to attrition of crystals by forced collision against solid surface areas or by treatment in the equipment. Such solid surface areas may be moving parts, for instance stirrer blades or crushing plates, or may have fixed positions, such as, for instance, collision plates, baffles, etc.
The present invention, in particular, relates to a continuous process for preparing of ammonium sulphate large crystal products.
Such process, particularly for the production of ammonium sulphate crystal products, is described in Japanese patent application JP 2000 072436 ('436). This document describes continuous crystallization of ammonium sulphate in a stirred crystallization vessel, for instance, in a DTB (Draft Tube Baffled) crystallizer (as shown in FIG. 1 of said document), and is aimed at the production of big and rough crystals of ammonium sulphate. It is to be noted, that in the DTB crystallizer embodiments of '436 only small crystals are brought into (external) circulation through a heat exchanger after classifying as to crystal size in a classifying zone, and water is added to ensure dissolution of the small crystals. In '436 control of slurry properties in the external circulation circuit (comprising a pump) is done by determining the particle size distribution (PSD) in the crystallizer, or of the slurry concentration in the external circulation line, by means of sampling at regular intervals of time. It is to be noted, that the mechanism shown in '436 works for DTB-type crystallizers, but is not applicable in Oslo-type crystallizers for obtaining a stable average crystal size (in particular as to crystal diameter). It is, however, a disadvantage of the method of '436 that undesirable strong fluctuation of particle size distribution occurs over time. Use of a DTB crystallizer for the production of large crystals is also described in, for instance, WO 01/91874 ('174) or WO 93/19826 ('826). Control of the operation of such crystallizer in '174 or '826 is, similarly to the '436 process, achieved by means of dissolution of fine crystals in the external circulation circuit in order to obtain larger crystals on average, while keeping the weight of crystallized material in the crystallizer at a constant level. It is further to be noted, that '174 incorrectly also mentions that the process of '174 also could be operated in fluid bed crystallizers. Such process, however, then still would require the dissolution of fine crystals in the external circulation circuit.
Another process for the production of ammonium sulphate crystal products is known from JP-A-63103821. This document describes a complicated process for the production of ammonium sulphate large crystal products wherein the share of ammonium sulphate crystals above a predetermined crystal size is increased by adding fine ammonium sulphate crystals, i.e. crystals below a predetermined size, to the suspended crystal mass as seed crystals. In the said known process the fine ammonium sulphate crystals are obtained in a process which involves withdrawing a product slurry which comprises the ammonium sulphate solution from the crystallization vessel, separating the ammonium sulphate product crystals from the ammonium sulphate solution, for instance by using a centrifuge, drying the separated ammonium sulphate product crystals, and size classifying the dried ammonium sulphate product crystals to obtain a fraction of ammonium sulphate product crystals above a predetermined size and a fraction of fine ammonium sulphate crystals below a predetermined size. Thus, the fine ammonium sulphate crystals are obtained by separating them as such from the final product. The dried fine ammonium sulphate crystals are then added to the suspended crystal mass in the crystallization vessel by means of a dosing apparatus with which the rate at which the dried fine ammonium sulphate crystals are added may be varied and controlled by adjusting the opening and closing time of the dosing apparatus. In the said known process the need for fine ammonium sulphate crystals in the suspended crystal mass is determined by determining the size distribution of the dried ammonium sulphate product crystals. It is described that the addition of the fine ammonium sulphate product crystals is preferably started at the moment that the share of crystals above the predetermined size starts to increase and that the addition is stopped when this share has reached a maximum and start to decline. Although in the process of '821 an external circulation circuit may be present for ensuring heat transfer into the crystallization vessel, there is no controlling of properties of the slurry in the external circulation circuit.
A major disadvantage of this known process is that the dried fine ammonium sulphate crystals tend to stick to each other, if no special precautions are taken to avoid contact of the fine ammonium sulphate crystals with moisture. Moreover, dosing of fine ammonium sulphate crystals and varying the rate at which fine ammonium sulphate crystals are added, is cumbersome due to such sticking to each other. Furthermore, clusters of fine ammonium sulphate crystals, which are sticking to each other, are not effective as seed crystals.
It is a goal of the invention to provide a process for obtaining large crystal products, having stable average crystal size at a predetermined large value of average crystal diameter, without problems of sticking, and wherein large crystal products are produced continuously in highest possible amounts, so that—if necessary by means of a subsequent sieving step—high yields of large crystal product meeting market specifications can be obtained. More particularly, the aim of the present invention is to provide a continuous process for preparing large crystal products of a large crystal product in a fluid bed crystallizer, said fluid bed crystallizer comprising a crystallization vessel and means for mechanically impacting of crystals; and connected to the crystallization vessel (i) a feed line; (ii) an external circulation circuit comprising means for determining properties of the slurry in the external circulation circuit and a heat exchanger; and (iii) a product withdrawal line.
Surprisingly, this goal is achieved in that for any specific crystalline product and at any flow rate chosen in the external circulation circuit, the weight percentage of crystals of the crystalline product in the slurry in the external circulation circuit before the heat exchanger, is controlled by said means for determining properties of the slurry within a specified and predetermined narrow range having a maximum and minimum value not more apart than at most 25% by weight and falling within the range of from 1 to 50% by weight. As meant herein, the term “controlled” means that the actually obtained results from the determination of properties of crystals in the external circulation circuit before the heat exchanger are used to establish the weight percentage of such crystals at any time during the process, and to take action for keeping said weight percentage within a predetermined range. The control, by methods explained below in more detail, thereby ensures that the said weight percentage will not go beyond such predetermined range.